Conventionally, electrolyte solutions in which lithium salts are dissolved in nonaqueous solvents have been used as electrolyte solutions for lithium secondary batteries. As lithium salts, substances such as LiPF6 and LiBF4 are generally known. Further, development of lithium secondary batteries using various kinds of lithium salts has been actively conducted currently.
For example, Patent Document 1 discloses a lithium secondary battery wherein an imide series lithium salt represented by a formula: Li(CnX2n+1Y)2N (“X” is halogen, “n” is an integer from 1 to 4, and “Y” is a CO group or a SO2 group) is dissolved in a nonaqueous solvent. Further, Patent Document 2 discloses a lithium secondary battery wherein an imide series lithium salt represented by a formula: LiN(CmF2m+1SO2) (CnF2n+1SO2) (“m” and “n” are each an independent integer from 1 to 4) is dissolved in a nonaqueous solvent. However, these lithium salts have a problem that their Li ionic transference numbers are low because they have only one Li cation contained in their respective molecular structures.
On the other hand, Patent Document 3 discloses a lithium secondary battery using a nonaqueous solvent wherein one or both of LiPF6 or LiBF4, and at least one substance from Li3PO4, Li2 (OH)3 PO4, Li (CH3)2PO4, and Li2 (C2H5) PO4 are comprised as electrolytes. However, lithium phosphate salts having plural Li ions in their respective molecular structures as the case of Li3PO4 have a problem in general that their solubility to a solvent is low. Thus, it has been difficult to sufficiently improve their Li ionic transference number.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 7-85888    Patent Document 2: JP-A No. 2001-68154    Patent Document 3: JP-A No. 10-189043